This invention relates generally to differential-pressure transmitters adapted to produce a signal proportional to a process variable, and more particularly to a transmitter whose differential-pressure sensors are insensitive to the temperature of the process fluid.
A differential-pressure transmitter is used to measure various process variables, such as fluid flow rate, liquid level and density. Such transmitters are widely used in industrial process systems to produce an output signal suitable for transmission to a remote station for operating indicator or control mechanisms. While the present invention will mainly be described in connection with the measurement of liquid level, it is to be understood that it is also applicable to the measurement of other process variables such as flow rate.
Liquid level transmitters are known which operate on the force-balance principle, the transmitter being directly bolted onto the tank containing the liquid. The fluid level in either an open tank or a closed tank under pressure or vacuum is detected by a differential-pressure capsule assembly which senses the difference between the weight of the liquid (level) on both sides of the capsule, and converts it into a force that is transmitted by a connecting rod to the lower extremity of a force beam.
In existing types of liquid level transmitters, the differential capsule assembly includes a pair of coupled diaphragms which define a fill space containing a hydraulic fluid that provides a non-compressible back up under high static pressures. The outer diaphragm is exposed to the liquid in the tank, whereas the inner diaphragm is exposed to atmospheric or relatively low pressures. The coupled diaphragms are linked to the lower extremity of the force beam by a connecting rod passing through a support tube which joins the housing of the capsule assembly with the body of the force balance meter, the support tube defining the low pressure process chamber.
Thus the force on the diaphragms in response to the difference in pressure is transferred by the rod to the beam to deflect the beam accordingly. The force mechanically applied to the beam is converted by the force balance transducer into a signal proportional thereto. Such mechanical coupling between the differential capsule assembly and the force beam gives rise to a serious source of error.
The metal connecting rod extending between the diaphragms and the force beam is disposed within the low pressure process chamber defined by the metal support tube joining the housing of the capsule assembly to the meter body. The temperature of this rod is often at a temperature which is distinctly at variance with the temperature of the support tube. The reason for this is that the support tube responds not only to the process temperature to which the rod is also exposed, but to ambient temperatures as well. When, therefore, the rod and the support tube are subjected to different temperatures, the mechanical coupling between the diaphragms and the force beam will change length due to the coefficient of expansion of the metals. This change in length, multiplied by the system gradient, generates a force which, when compared to the full scale operating force, will determine the magnitude of temperature error.
In order to minimize the influence of temperature on the coupling in a liquid level transmitter between the level sensor and the meter body, it is known to replace the mechanical coupling with a hydraulic coupling in the form of a flexible capillary tube extending between the fill space in the sensor assembly and a measuring diaphragm in the meter, this diaphragm being linked to the force beam. With a hydraulic coupling of this type, changes in the length of the capillary relative to that of the support tube have virtually no effect on the performance of the coupling. But should the process heat up the fill between the diaphragms of the differential assembly, the resultant expansion in the fill will generate a pressure proportional to the fluid volume and inversely proportional to the diaphragm compliance, thereby simulating a change in liquid level and producing a spurious signal.
In the above-identified copending application, there is disclosed a liquid level transmitter having a sensor including a hydraulic fill chamber whose physical dimensions are varied as a function of temperature to alter the internal volume of the chamber in proportion to changes in fill pressure occurring by reason of changes in fill temperature, thereby rendering the sensor insensitive to temperature variations.
In the sensor disclosed in the copending application, the fill chamber is defined by a sensing diaphragm which is exposed to liquid in the tank and an insert incorporated in the sensor housing and spaced from the diaphragm. The sensor housing is joined by a support tube to the body of a meter whose pivoted beam is mechanically linked to a measuring diaphragm disposed in a diaphragm chamber. The fill chamber of the sensor communicates with the diaphragm chamber of the meter body by way of a capillary extending through the coupling tube. The fill chamber, the capillary and the diaphragm chamber contain hydraulic fluid, whereby the pressure of tank liquid imposed on the diaphragm of the sensor is transmitted hydraulically to the measuring diaphragm to apply a torque to the force beam. In order to compensate for the effect of temperature on the volume of the fill in the fill chamber of the sensor, the insert is formed of a metal having a low coefficient of expansion relative to that of the sensor housing, whereby the volume of the fill chamber varies in proportion to changes in the volume of the fill.
In the liquid level transmitter disclosed in the above-identified copending application, the body of the force-balance transducer or meter is coupled to the temperature-compensated high pressure sensor through a capillary extending through a short rigid support tube, whereas the low pressure sensor is incorporated in the body of the meter and is not temperature-compensated.
Where the liquid in the tank whose liquid level is being measured is at an exceptionally high temperature (i.e., 600.degree. F) or at any exceptionally low temperature (i.e., -40.degree. F), the transmitter arrangement disclosed in the copending application is not suitable, in that the meter, because of its close proximity to the tank, is affected by the temperature of the liquid. Because the low pressure sensor in the body of the meter is no longer at normal atmospheric temperature and lacks temperature compensation, the resulting liquid level reading is inaccurate.